Descending nerve tracts in the spinal cord of the rat. I. Fibers from the midbrain

Unilateral stereotaxic lesions were made in the superior colliculus, the inferior colliculus, the red nucleus or the reticular formation of the midbrain of rats. The descending fibers from these sites were studied using the method of Nauta ('57). One crossed and two uncrossed tracts descend from the superior colliculus. These tracts terminate mainly in the reticular formation of the medulla oblongata. Only a small number of tecto‐spinal fibers were found. These were confined to the upper contralateral cervical segments of the cord. The tecto‐spinal fibers terminate on cells in the ventral gray matter of the cord, corresponding in position to those in Rexed's lamina VIII in the cat. One uncrossed descending tract passes from the inferior colliculus and terminates in the reticular formation of the medulla oblongata. No fibers were contributed to the tecto‐spinal tract by the inferior colliculus. Rubro‐spinal fibers decussate completely in the ventral tegmental decussation and descend in the lateral funiculus of the cord into lumbar segments and on interneurons which correspond in position to those in Rexed's laminae V and VI in the cat. From the reticular formation of the midbrain, ipsilateral fibers course in the ventral funiculus of the cord, descending to the ninth or tenth thoracic segments. These fibers terminate on cells corresponding in position to those in Rexed's lamina VIII in the cat. Considerable preterminal degeneration, mainly ipsilateral, was found in the brain stem of the animals with lesions in the midbrain reticular formation. The findings are discussed in relation to what has been reported previously in the rat, and in other animals.